Exhaust gas purification catalyst

ABSTRACT

A substrate ( 11 ) of an exhaust gas purification catalyst ( 10 ) includes inflow-side cells ( 21 ), outflow-side cells ( 22 ), and porous partition walls ( 23 ), each separating the inflow-side cell and the outflow-side cell. Catalyst portions ( 14, 15 ) are provided on the surfaces of the partition walls that each face the inflow-side cell and/or the surfaces of the partition walls that each face the outflow-side cell. In a cross section vertical to an exhaust gas flow direction, the percentage of the total area of voids, each void satisfying the expression L/{2(πS) 1/2 }≤1.1 (wherein L is the perimeter of the void in the cross section, and S is the area of the void in the cross section), is greater than 10% to 30% or less based on the apparent area of the catalyst portion present on the partition wall. The content of zirconium element in terms of oxide (amount of ZrO2) in the catalyst portions is from 35 mass % to 85 mass %.

TECHNICAL FIELD

The present invention relates to exhaust gas purification catalysts.

BACKGROUND ART

Fuel economy standards for internal combustion engines, such as gasolineengines, are becoming stricter by the year, and gasoline directinjection engines (hereinbelow also called “GDI engines”) are now widelyused in order to comply with such standards. It is known that GDIengines have good fuel efficiency and provide high output, and on theother hand, it is also known that they emit more than 5 to 10 times theamount of particulate matter (hereinbelow also called “PM”, includingsoot) in exhaust gas compared to conventional port injection engines. Inview of complying with environmental regulations regarding PM emission,there are demands that gasoline engine vehicles, such as GDI enginevehicles, should be equipped with gasoline particulate filters(hereinbelow also called “GPFs”) having the capability to collect PM, asin diesel engine vehicles.

Typically, there is limited space for installing exhaust gaspurification catalysts. Thus, a filter catalyst has come into use inrecent years that includes a filter described above and noble-metalthree-way catalysts, such as Pd, Pt and Rh, supported on the filter tocollect PM and also remove pollutants such as nitrogen oxides (NOx),carbon monoxide (CO) and hydrocarbons (HC).

For example, Patent Literature 1 discloses an exhaust gas purificationfilter arranged in an exhaust path of an internal combustion engine andcollecting particulate matter in exhaust gas exhausted from the internalcombustion engine, the exhaust gas purification filter including asubstrate and a catalyst layer provided on the substrate.

The catalyst layer contains a carrier and a metal catalyst; and, whenthe area of the catalyst layer is defined as 100% in an electronmicroscope observation image of a cross section of the catalyst layer,large pores having a circle equivalent diameter of greater than 5 μmoccupy 45% or more of the area.

CITATION LIST Patent Literature

Patent Literature 1: US 2019/299139A1

SUMMARY OF INVENTION Technical Problem

Conventionally, GPFs are divided into two types: in-wall types, in whichsubstantially the entire catalyst layer is formed within the partitionwall, and on-wall types, in which at least a portion of the catalystlayer is formed on the partition wall. On-wall types have better PMcollection performance than in-wall types; however, because of the highporosity of the partition wall in a GPF filter substrate, theadhesiveness of the on-wall catalyst layer to the partition wall isweaker compared to in-wall types, in which a catalyst layer is formedwithin the partition wall of the filter substrate. This results in atendency for on-wall catalyst layers to peel. Furthermore, GPFs stillhave issues in achieving both PM collection and suppression ofpressure-loss, regardless of whether the GPFs are of the in-wall oron-wall type.

The aforementioned conventional art attempts to overcome the issue ofachieving both PM collection and suppression of pressure-loss byforming, on a partition wall, a catalyst layer having many relativelylarge pores in percentage terms. In the literature, however, featuresfor suppressing peeling of the catalyst layer is not taken intoconsideration.

The present invention aims at solving the aforementioned and otherproblems of the conventional art, and providing an exhaust gaspurification catalyst capable of delivering high PM collectionperformance while suppressing peeling of the catalyst layer and anincrease in back pressure.

Solution to Problem

As a result of diligently studying features for suppressing peeling ofthe catalyst layer while delivering PM collection performance andsuppressing pressure loss, the inventors have found that theaforementioned issues can be overcome by adjusting the percentage ofnear-perfect circular pores within a specific range in a cross sectionof the catalyst layer, and also using zirconium in the catalyst layer ina specific amount.

The present invention has made based on the aforementioned finding, andprovides an exhaust gas purification catalyst including: a substrate,and catalyst portions provided in the substrate. The catalyst portionsinclude a plurality of voids. The substrate includes: inflow-side cells,each inflow-side cell being a space having an open end on an inflow sidethereof and a closed end on an outflow side thereof in an exhaust gasflow direction; outflow-side cells, each outflow-side cell being a spacehaving a closed end on an inflow side thereof and an open end on anoutflow side thereof in the exhaust gas flow direction; and porouspartition walls, each separating the inflow-side cell and theoutflow-side cell. The catalyst portions are provided on surfaces of thepartition walls that each face the inflow-side cell and/or surfaces ofthe partition walls that each face the outflow-side cell. In a crosssection vertical to the exhaust gas flow direction, the percentage ofthe total area of the voids, each void satisfying the expressionL/{2(πS)^(1/2)}≤1.1, wherein L is the perimeter of the void in the crosssection and S is the area of the void in the cross section, is greaterthan 10% to 30% or based on an apparent area of the catalyst portionpresent on the partition wall. The content of zirconium element in termsof oxide (amount of ZrO₂) in the catalyst portions is from 35 mass % to85 mass %.

Advantageous Effects of Invention

The present invention can provide an exhaust gas purification catalysthaving a wall-flow structure and delivering excellent PM collectionperformance while suppressing pressure loss and peeling of the catalystlayer. The present invention can also provide a method for manufacturingthe exhaust gas purification catalyst in an industrially advantageousmanner.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic cross-sectional perspective view of an exhaust gaspurification catalyst of one embodiment of the present invention.

FIG. 2 is a cross-sectional view of the portion surrounded by therectangle in FIG. 1 , when viewed along the axial direction of thesubstrate.

FIG. 3 is a diagram illustrating an exemplary method for extracting asample for use in observing cross sections of catalyst layers.

FIG. 4 is an example of an image of a catalyst portion on a partitionwall as observed under a scanning electron microscope with provisionalboundary lines drawn on the image.

FIG. 5 is an example of an image of the catalyst portion on thepartition wall as observed under a scanning electron microscope withdivision lines drawn on the image.

FIG. 6 is a schematic diagram for explaining a method for measuring therate of approximately-perfect circular voids.

FIG. 7 is an enlarged diagram of the portion surrounded by the rectanglein FIG. 2 .

FIG. 8 is a diagram of a catalyst layer obtained by partially modifyingthe catalyst layer shown in FIG. 7 .

DESCRIPTION OF EMBODIMENTS

The present invention is described below by way of preferred embodimentsthereof. The present invention is, however, not limited to the followingembodiments.

An example of the exhaust gas purification catalyst 10 according to thepresent embodiment is illustrated in FIGS. 1, 2, 7 and 8 .

The exhaust gas purification catalyst 10 is provided in an exhaust pathof an internal combustion engine such as a gasoline engine, andparticularly a GDI engine of a vehicle. The exhaust gas purificationcatalyst 10 is used, for example, as a GPF.

As illustrated in FIG. 1 , the exhaust gas purification catalyst 10includes a substrate 11 having a so-called wall-flow structure. Asubstrate made of any types of materials can be used as the substrate11. For example, a substrate made of ceramics, such as cordierite orsilicon carbide (SiC), can be favorably used. Typically, the substratehas a columnar shape as illustrated in FIG. 1 , and is arranged in theexhaust path of an internal combustion engine such that the axialdirection of the columnar shape substantially matches the exhaust gasflow direction X. FIG. 1 illustrates a substrate having a circularcylindrical outer shape. The outer shape of the substrate as a whole,however, may be an elliptic cylindrical or polygonal columnar shape,instead of a circular cylindrical shape.

As illustrated in FIG. 1 , the substrate 11 includes inflow-side cells21 and outflow-side cells 22. Each inflow-side cell 21 is a space, thespace extending in the exhaust gas flow direction X and having an openend on the inflow side thereof and a closed end on the outflow sidethereof in the direction X. Each outflow-side cell 22 is a space, thespace extending in the flow direction X and having a closed end on theinflow side thereof and an open end on the outflow side thereof in thedirection X.

The inflow-side cell 21 is closed by a sealing portion 24 at the end onthe exhaust gas outflow-side in a downstream end portion R2 in theexhaust gas flow direction X, but is open at the end on the exhaust gasinflow-side in an upstream end portion R1. The outflow-side cell 22 isclosed by a sealing portion 25 at the end on the exhaust gas inflow-sidein the upstream end portion R1, but is open at the end on the exhaustgas outflow-side in the downstream end portion R2. The inflow-side cell21 and the outflow-side cell 22 are configured such that a gas, aliquid, and the like can flow through an opening end (hereinafter alsoreferred to as “opening”), but the flow of exhaust gas is blocked at thesealing portion 24 and the sealing portion 25, which are closed portion.

The inflow-side cell 21 and the outflow-side cell 22 are each a bottomedcylindrical space extending in the axis direction of the substrate 11.The cross-sectional shape of each of the inflow-side cell 21 and theoutflow-side cell 22 in a cross section vertical to the axis directionof the substrate 11 may be any geometric shape, and examples thereofinclude a quadrilateral such as a square, a parallelogram, a rectangle,and a trapezoid, a polygon such as a triangle, a hexagon, and anoctagon, a circular shape, and an elliptic shape.

A porous partition wall 23 is formed between the inflow-side cell 21 andan adjacent outflow-side cell 22, the partition wall separating the twocells. The partition wall 23 serves as a side wall of the inflow-sidecell 21 and the outflow-side cell 22 having a bottomed cylindricalshape. The partition wall 23 has a porous structure to allow a gas suchas exhaust gas to pass therethrough. The thickness of the partition wall23 is preferably 150 μm to 400 μm, for example. As used herein, the term“thickness” refers to the thickness of a thinnest portion if thepartition wall 23 between the inflow-side cell 21 and the outflow-sidecell 22 does not have a uniform thickness.

In the substrate 11, the area of the opening of a single inflow-sidecell 21 in the inflow-side end portion R1 may be the same or differentfrom the area of the opening of a single outflow-side cell 22 in theoutflow-side end portion R2. Herein, the area of an opening refers tothe area on a plane vertical to the exhaust gas flow direction.

The substrate 11 supports catalyst portions including a catalyticallyactive component. In view of PM collection performance and exhaust gaspurification performance, the catalyst portions preferably include firstcatalyst portions 14 in the form of a layer (hereinafter also called“first catalyst layer(s) 14”), the first catalyst portions 14 each beingprovided at least on the upstream side in the exhaust gas flow directionX on the surface of the partition wall 23 that faces the inflow-sidecell 21, and second catalyst portions 15 in the form of a layer(hereinafter also called “second catalyst layer(s) 15”), the secondcatalyst portions 15 each being provided at least on the downstream sidein the exhaust gas flow direction X on the surface of the partition wall23 that faces the outflow-side cell 22, as shown in FIG. 2 . In view ofPM collection performance and exhaust gas purification performance, thefirst catalyst layer 14 is preferably formed in at least a portion inthe area extending from the upstream end by a length of 20 mm downstreamtherefrom in the exhaust gas flow direction. In view of PM collectionperformance and exhaust gas purification performance, the secondcatalyst layer 15 is preferably formed in at least a portion in the areaextending from the downstream end by a length of 20 mm upstreamtherefrom in the exhaust gas flow direction. Hereinbelow, the “catalystportion” is also referred to as “catalyst layer”, and the “catalystlayer” may refer to either the first catalyst layer 14 or the secondcatalyst layer 15. Typically, the length of the exhaust gas purificationcatalyst in the exhaust gas flow direction X is from 50 to 200 mm.

The inventors have found that peeling of the catalyst layer can besuppressed effectively when the catalyst layer on the partition wallincludes at least a certain quantity of circular voids in a crosssection vertical to the exhaust gas flow direction of the catalyst 10.The reason for this is considered as follows.

Voids in the catalyst layer are typically formed by adding a voidforming reagent to a catalyst layer-forming slurry, and calcinating theslurry to eliminate the void forming reagent. The slurry often includesa void forming reagent having a greater diameter than the diameter ofpores in the partition wall. Thus, many particles of the void formingreagent are present in a state of abutting the partition wall's surface,without entering the pores in the partition wall. In a cross-sectionalview of the catalyst 10, circular voids have a smaller point of contactwith the partition wall (substrate) compared to voids having othershapes, such as acicular or rectangular voids. Further, sphericalparticles of a void forming reagent, which form circular voids in thecross section of the catalyst 10, have a small surface area per unitvolume, and are thus less likely to aggregate in the slurry compared toan acicular void forming reagent. This inhibits the formation of hugevoids in the catalyst layer. For the reasons above, the presence of acertain quantity of circular voids in the cross section of the catalyst10 contributes to suppression of peeling of the catalyst layer. Thus, acatalyst layer including a certain quantity of circular voids in thecross section of the catalyst 10 can be effectively prevented frompeeling between the catalyst layer and the partition wall (substrate)while suppressing pressure loss. Stated differently, the catalyst 10 ofthe present embodiment can exhibit the conventional voids' effect ofsuppressing pressure loss by including voids in the cross section of thecatalyst 10, and also, the catalyst 10 can further exhibit, by formingthe voids in a circular shape, the effect of effectively suppressingpeeling between the catalyst layer and the partition wall (substrate),which would otherwise be caused by the voids. The effects of the presentinvention are effective in uses for GPFs, which particularly require thesuppression of pressure loss by forming voids in the catalyst layerwhile maintaining high collection performance.

More specifically, in a cross section of the exhaust gas purificationcatalyst 10 vertical to the exhaust gas flow direction (hereinbelow alsocalled “direction X” or “exhaust gas flow direction X”), the percentageof the total area of the voids, each void satisfying the expressionL/{2(πS)^(1/2)}≤1.1 (wherein L is the perimeter of the void in the crosssection, and S is the area of the void in the cross section) (thepercentage is hereinafter also referred to as “circular voidpercentage”), is preferably greater than 10% to 30% or less based on theapparent area of the catalyst layer present on the partition wall 23.

The apparent area of the catalyst layer present on the partition wall 23in the aforementioned cross section refers to the entire area of thecatalyst layer, including the voids, present on the partition wall 23 inthe aforementioned cross section.

The closer the value of the expression L/{2(πS)^(1/2)} is to 1, thecloser the shape is to a perfect circle. The minimum of the value fromthe expression is 1. Thus, the shape of a void whose value derived bythe expression is 1.1 or less is close to a perfect circle. Hereinbelow,a void satisfying the aforementioned expression is called an“approximately-perfect circular void”. Considering that the circularvoid percentage is determined by image processing, theapproximately-perfect circular voids preferably have a circle equivalentdiameter of 1 μm or greater. Thus, more specifically, the circular voidpercentage is preferably the percentage of the total area of voids eachsatisfying the aforementioned expression and having a circle equivalentdiameter of from 1 to 60 μm. Herein, “circle equivalent diameter” refersto the diameter of a circle having the same area as the void.

The circular void percentage higher than 10% further reduces pressureloss. The circular void percentage to 30% or less suppresses peeling ofthe catalyst layer from the partition wall and also suppressesdeterioration in PM collection performance. From these viewpoints, thecircular void percentage of the catalyst layer on the partition wall ismore preferably from 11 to 20%, even more preferably from 12 to 15%. Inthe present embodiment, even though the circular void percentage is ashigh as above 10%, peeling of the catalyst layer from the partition wallis prevented by controlling the content of zirconium element in terms ofoxide (amount of ZrO₂) in the catalyst layer to the range describedlater. The reason why an increase in the content of ZrO₂ in the catalystlayer favorably suppresses peeling of the catalyst portion is probablybecause of characteristics of ZrO₂, including the high melting point;large bond-dissociation energy with oxygen; high flexural strength andfracture toughness, which lead to durability to mechanical action; andsmall change in specific surface area before and after calcinating.

The circular void percentage is preferably determined throughobservation with a scanning electron microscope. The position, in thedirection X, of the cross section of the exhaust gas purificationcatalyst 10 to be observed with a scanning electron microscope is notparticularly limited, and a cross section at any discretionary positioncan be observed. An example of a method for determining the circularvoid percentage is described below.

(1) Sampling:

As illustrated in FIG. 3(a), a circular-cylindrical sample S having adiameter of 25.4 mm and a central axis parallel to the exhaust gas flowdirection is cut out from the exhaust gas purification catalyst 10. Thelength of the circular-cylindrical sample S spans the entirety of theexhaust gas purification catalyst 10 in the exhaust gas flow directionX. In a plane (see FIG. 3(b)) vertical to the exhaust gas flowdirection, the position at which the circular-cylindrical sample S iscut out from the exhaust gas purification catalyst 10 is notparticularly limited. In the aforementioned plane, the central axis ofthe cut-out portion is preferably located at a position radially outwardfrom the substrate's center C and separated therefrom by a length of 10to 70% based on the radial length, as illustrated in FIG. 3(b), in viewof allowing a plurality of circular-cylindrical samples S to be cut outfrom a single catalyst. The “substrate's center” in the aforementionedplane is a point that divides, into two equal parts, the longest linesegment traversing the outer contour of the substrate in the plane. The“radial length” is half the aforementioned longest line segment.

As described above, the position of the cross section of the exhaust gaspurification catalyst 10 in the exhaust gas flow direction X is notparticularly limited. For example, the cross section of the firstcatalyst layer or the second catalyst layer can be easily observed ifthe cross-sectional position is at a position on the upstream side thatis within 10 mm from the upstream end, in the direction X, of thecircular-cylindrical sample S and that does not overlap the sealingportion 25, or at a position on the downstream side that is within 10 mmfrom the downstream end, in the direction X, of the sample and that doesnot overlap the sealing portion 24. However, the observation surface maybe in a central portion, in the direction X, of the sample S. Theobservation surface is exposed by cutting the sample at a cross sectionvertical to the direction X. The observation surface is embedded with aresin and polished. In view of handling of the sample, the thickness(i.e., length in the direction X) of the observation sample ispreferably 10 mm. FIG. 3(c) illustrates the case of obtaining: twosamples for observing at positions t and b located 10 mm away from theupstream and downstream ends, respectively, in the exhaust gas flowdirection X (samples T and B in FIG. 3(c)); and a sample for observingat the center position m in the exhaust gas flow direction X (sample Min FIG. 3(c)).

(2) Determination of Partition Wall's Outer Edge in Contact withCatalyst Layer:

The distribution of partition wall components and catalyst layercomponents on the aforementioned observation surface is observed anddetermined by electron probe micro-analyzer (EPMA) mapping. Theobservation magnification is from 40× to 1000× (acceleration voltage: 15to 25 kV). The substrate components and catalyst layer components to beused for determining the area of the catalyst layer on the partitionwall can be analyzed by subjecting the substrate and the catalyst layerof the exhaust gas purification catalyst 10 to X-ray fluorescenceanalysis etc. For example, in cases where the substrate includescordierite, the distribution of Si or Mg as a substrate component isobserved. Examples of catalyst layer components include Al, Ce, Zr, Pt,Pd, and Rh. The same sample is subjected to image capturing at the sameposition with a scanning electron microscope (SEM) under the samemagnification as for EPMA mapping, and the image is matched with theaforementioned EPMA mapping image, to determine, on the SEM image, thearea in which the catalyst layer components are distributed and the areain which the partition wall components are distributed. Then, asillustrated in FIG. 4 , for example, the outer edge of the area in whichthe partition wall components are distributed is defined as a“provisional boundary line” of the catalyst layer on the partition wall.The acceleration voltage of SEM observation is preferably from 10 to 15kV.

In the SEM observation image and the EPMA observation image, thelongitudinal direction of the observation image (or, in cases where theobservation image is square, the direction of one side) is preferablysubstantially vertical to the thickness direction of the catalyst layer.An image in which the catalyst layer's outer edge (i.e., the outer edgeon the other side than the partition wall side) extends along thelongitudinal direction of the observation image (or the direction of oneside in cases where the observation image is square) over the entiretyof the observation image in the longitudinal direction is selected asthe SEM observation image and the EPMA observation image of the catalystlayer. In other words, for example, there may be a case where thecatalyst layer exists only in a part of the observation image in thelongitudinal direction because the catalyst layer's outer edge is bentin the middle of the longitudinal direction of the observation image,and such an image is not selected. This is because such bent sectionshave poor exhaust gas permeability and little contribution to pressureloss.

(3) Determination of Width of Division:

The aforementioned outer edge of the area in which the partition wallcomponents are distributed in the EPMA mapping is defined on the SEMimage as the outer edge, on the partition wall side, of the catalystlayer on the partition wall. On the other hand, the catalyst layer'souter edge on the other side than the partition wall side is defined onthe basis of a difference in color between the catalyst layer/substrateand the other sections. These outer edges are defined as “provisionalboundary lines”, as illustrated in FIG. 4 , for example. An exemplarycolor difference is illustrated in FIGS. 4 and 5 , and in FIGS. 4 and 5, the catalyst layer and substrate are white or gray, whereas the othersections are black. The “provisional boundary lines” can be defined byimage processing software for drawing boundary lines, which will bedescribed later, and the selective threshold can be set within the samerange as described later. The “provisional boundary lines” are fordefining the later-described width of the division and for provisionallydefining approximately-perfect circular voids, but are not to be usedfor measuring the area of the catalyst layer.

In the catalyst layer on the partition wall as defined by theprovisional boundary lines, the void area (S) and void perimeter (L) foreach void are measured, and it is determined for each void whether ornot the circle equivalent diameter is from 1 to 60 μm and whether or notthe void satisfies the expression L/{2(πS)^(1/2)}≤1.1. Image processingfor determining the shape of each void can be performed with imageprocessing software for drawing boundary lines. Examples of such imageprocessing software for drawing boundary lines include PictBear (fromFenrir Inc.). The selective threshold is preferably from 20 to 40 withreference to, for example, the color of a clear void section. The “colorof a clear void section” is the color of a section other than theconstituent components of the catalyst layer or the substrate, and istypically black, as illustrated in FIGS. 4 and 5 . Preferably, the outerperiphery of each void is drawn by a line with a thickness of at least20 points. After drawing the boundary line, the perimeter L and area Sof each void are calculated. Image analysis software can be used for thecalculation, including ImageJ (public domain), Photoshop (from AdobeSystems Incorporated), and AreaQ (from S-Tech Corporation). In a totalof 20 fields-of-view, the area is measured for each void whose entiretyis included within the catalyst layer on the partition wall as definedby the aforementioned “provisional boundary lines”, whose circleequivalent diameter is from 1 to 60 μm, and that satisfies theexpression L/{2(πS)“²}≤1.1. For all such voids, the respective circleequivalent diameters are measured. Then, as illustrated in FIG. 5 , forexample, a plurality of straight lines (hereinbelow also called“division lines”) that are parallel to the catalyst layer's thicknessdirection are drawn on the SEM image of the catalyst's cross section atintervals of a width that is equivalent to the median diameter of theobtained circle equivalent diameters (hereinafter also called “width ofthe division”). Each of the 20 fields-of-view is a field-of-viewincluding at least one void whose entirety is included within thecatalyst layer on the partition wall as defined by the aforementioned“provisional boundary lines”, whose circle equivalent diameter is from 1to 60 μm, and that satisfies the expression L/{2(πS)^(1/2)}≤1.1;fields-of-view that do not include such a void are excluded from the 20fields-of-view. There may be a case where an observation image showscatalyst layers respectively formed on both surfaces of a partitionwall, both such catalyst layers including at least one void that has acircle equivalent diameter of from 1 to 60 μm and satisfies theexpression L/{2(πS)”²}≤1.1. In such a case, the measurement result isobtained for the catalyst layer whose area (i.e., area surrounded by theprovisional boundary lines and the edges of the observation image) islarger among the two catalyst layers, and the other catalyst layer isexcluded from measurement.

(4) Determination of Apparent Area of Catalyst Layer on Partition Wall:

Measurement in the SEM observation image is performed in a field-of-viewin which there are from 30 to 40 division lines drawn at intervals ofthe aforementioned width of the division. In cases where this conditionis not satisfied, the magnification is changed before performing thefollowing process. As illustrated in FIG. 5 , a line is drawn byconnecting, with straight line segments, intersection points between therespective division lines and the partition wall's outer edge determinedby the aforementioned EPMA mapping, and this line is taken as the linedefining the boundary between the partition wall and the catalyst layer(this line is called a “partition wall boundary line” and distinguishedfrom the aforementioned “provisional boundary line”). Further, on theother side than the partition wall side, a line is drawn by connecting,with straight line segments, intersection points between the respectivedivision lines and the edge of the catalyst layer as determined by theaforementioned color difference, and this line is taken as the linedefining the boundary between the catalyst layer and the exterior (thisline is called a “catalyst layer boundary line” and distinguished fromthe aforementioned “provisional boundary line”).

The apparent area of the catalyst layer is determined as the areasurrounded by the partition wall boundary line, the catalyst layerboundary line, and the two outermost division lines when viewed in thelongitudinal direction (the thick division lines in FIG. 5 ) in theobservation image, as illustrated in FIG. 5 , for example.

(5) Determination of Area of Approximately-Perfect Circular Void:

Next, the region surrounded by the partition wall boundary line, thecatalyst layer boundary line, and the two outermost division lines whenviewed in the longitudinal direction (hereinafter also called ‘regionwith apparent area’) is re-determined as the region of the catalystlayer on the partition wall. Along with re-determining the region of thecatalyst layer on the partition wall, voids and respective areas thereofare also re-determined that are included within the newly-determinedregion with “apparent area”, instead of between the “provisionalboundary lines” as determined in (2) above, and that have a circleequivalent diameter of from 1 to 60 μm and satisfy the expressionL/{2(πS)^(1/2)}≤1.1. At this time, the following approximation process(see FIG. 6 ) is performed, which was not performed in theaforementioned process of “Determination of Width of Division”.

There may be substantially-circular voids that are partially protrudingor chipped and whose actual perimeter and area thus do not satisfy theexpression L/{2(πS)^(1/2)}<1.1 (e.g., α and β illustrated in FIG. 6(a)).Such substantially-circular voids are subjected to the followingprocess. If the ratio of the length of a dotted-line segment (i.e., thelength of the dotted line illustrated in FIG. 6(a)), which connects bothends of the perimeter excluding the protruding/chipped portion (i.e.,the solid line as illustrated in FIG. 6(a)), to the length of thesolid-line perimeter is 30% or less, then the range surrounded by thedotted line and the solid line is deemed as a void, and it is determinedwhether or not the area surrounded by the dotted and solid lines and theperimeter, which is the total length of the dotted and solid lines,satisfy the expression L/{2(πS)^(1/2)}≤1.1. If satisfied, the void isincluded in the calculation of the circular void percentage, by deemingthe segment surrounded by the solid and dotted lines as the shape of thevoid. FIG. 6(b) illustrates virtual void shapes when α and β have beendeemed as above. On the other hand, if the aforementioned ratio exceeds30%, the aforementioned process is not performed, and the area andperimeter are calculated according to the general method, to determinewhether the expression L/{2(πS)^(1/2)}≤1.1 is satisfied. Voids that arepartially chipped by the partition wall boundary line, the catalystlayer boundary line, or one of the two outermost division lines whenviewed in the longitudinal direction (e.g., γ and δ illustrated in FIG.6(a)) are subjected to the same process as that for α and β by using thevoid's perimeter excluding the boundary line (i.e., the length of thesolid line) and the boundary line segment of the void (i.e., the lengthof the dotted line). Voids partially chipped by division lines are alsosubjected to the same process as that for α and β.

According to the aforementioned process, the total area of theapproximately-perfect circular voids that exist within theaforementioned region with “apparent area” and have a circle equivalentdiameter of from 1 to 60 um is determined for the catalyst layer on thepartition wall, and the ratio of the total area to the apparent area ofthe catalyst layer on the partition wall is calculated. The ratio iscalculated for each of the fields-of-view, and the average value for 20fields-of-view is taken as the circular void percentage. If, as a resultof re-determining the aforementioned region, any of the fields-of-viewends up including no approximately-perfect circular void having a circleequivalent diameter of from 1 to 60 μm within the aforementioned regionwith “apparent area”, another field-of-view is taken anew. Then, in theother field-of-view, division lines are drawn at intervals of the widthof the division found as described above, and also, the region with“apparent area” is determined according to the aforementioned procedureusing the EPMA observation image. In this way, the average value iscalculated for 20 fields-of-view, each including at least oneapproximately-perfect circular void having a circle equivalent diameterof from 1 to 60 μm within the aforementioned region with “apparentarea”.

In the present invention, the circular void percentage, as obtainedthrough observing a total of 20 fields-of-view in at least oneobservation surface in a single sample S, satisfies the aforementionedrange. For example, 20 fields-of-view may be observed on each of aplurality of observation surfaces at different positions in the exhaustgas flow direction X. On this occasion, in a case where the circularvoid percentages vary on the different observation surfaces, it isconsidered that the requirement of the present invention is satisfied ifthe circular void percentage in any of the observation surfacessatisfies the aforementioned range. The same is also applied to thelater-described circle equivalent diameter, the number ofapproximately-perfect circular voids per 1 mm², and the ratio of thecatalyst portion's thickness to the thickness of the partition wall. Thefollowing feature (A) or (B) is further preferred.

(A) The circular void percentage satisfies the aforementioned range whenobserving 20 fields-of-view on either: an observation surface at aposition on the upstream side that is within 10 mm from the upstreamend, in the direction X, of the sample S and that does not overlap thesealing portion 25; or an observation surface at a position on thedownstream side that is within 10 mm from the downstream end, in thedirection X, of the sample S and that does not overlap the sealingportion 24.

(B) The circular void percentage satisfies the aforementioned range whenobserving a total of 20 fields-of-view, including 10 fields-of-view onthe observation surface at the aforementioned upstream position, and 10fields-of-view on the observation surface at the aforementioneddownstream position.

In the feature (B), the average value of the circular void percentagesof the 10 fields-of-view on the upstream side and the average value ofthe circular void percentages of the 10 fields-of-view on the downstreamside are both preferably greater than 10% to 30% or less, morepreferably from 11 to 20%, even more preferably from 12 to 15%.

In the catalyst layer on the partition wall as defined by the “partitionwall boundary line” and the “catalyst layer boundary line”, the averagevalue of the circle equivalent diameters of approximately-perfectcircular voids is from 1 to 60 μm. In this range, it is possible todetermine the percentage of the circular voids that contribute tosuppression of peeling, suppression of pressure loss, and improved PMcollection performance. In view of further suppressing peeling andpressure loss as well as improving PM collection performance, theaverage value of the circle equivalent diameters is preferably from 5 to50 μm, more preferably from 10 to 30 μm. The average value of the circleequivalent diameters is determined by first calculating, for eachfield-of-view, the average value of the circle equivalent diameters forthe approximately-perfect circular voids having a circle equivalentdiameter from 1 to 60 μm, and then averaging the average values for the20 fields-of-view ultimately used for determining the circular voidpercentage.

The average number of approximately-perfect circular voids having acircle equivalent diameter from 1 to 60 μm, per 1 mm² of apparent areaof the catalyst layer present on the partition wall, is preferably 250or greater in view of suppressing pressure loss, and is preferably 4000or fewer in view of PM collection performance and suppression ofpeeling. From these viewpoints, the average number ofapproximately-perfect circular voids having a circle equivalent diameterfrom 1 to 60 μm is more preferably from 400 to 2000, even morepreferably from 550 to 1000, per 1 mm² of the aforementioned apparentarea. The average number of approximately-perfect circular voids havinga circle equivalent diameter from 1 to 60 μm, per 1 mm² of apparent areaof the catalyst layer, is the average value for the 20 fields-of-viewultimately used for determining the circular void percentage.

The average thickness of the catalyst layer on the partition wall ispreferably 8% or greater based on the average thickness of the partitionwall, in view of PM collection performance, and preferably 20% or lessin view of suppressing pressure loss. From these viewpoints, the averagethickness of the catalyst layer on the partition wall is more preferablyfrom 9 to 16%, even more preferably from 10 to 12%, based on the averagethickness of the partition wall. The average thickness of the catalystlayer on the partition wall is determined in an observation image bydividing the apparent area of the catalyst layer on the partition wallby the distance between the two outermost division lines when viewed inthe longitudinal direction of the observation image. The averagethickness of the partition wall is determined in an observation image bydividing the area of a section surrounded by the partition wall's oneouter edge (i.e., the partition wall boundary line), the partitionwall's other outer edge (defined according to the same method as for thepartition wall boundary line) and the two outermost division lines whenviewed in the longitudinal direction of the observation image, by thedistance between the two outermost division lines when viewed in thelongitudinal direction of the observation image. In this way, the ratioof the thickness of the catalyst layer on the partition wall to thethickness of the partition wall is determined for each field-of-view.The average value of the thickness ratio is calculated from those of the20 fields-of-view ultimately used for determining the circular voidpercentage, and the resulting average value is taken as theaforementioned ratio. In view of PM collection performance andsuppression of pressure loss, the thickness of the partition wall ispreferably from 200 to 270 μm, more preferably from 210 to 260 μm.

The circular void percentage as described hereinbefore can be obtainedin the following manner: in the later-described suitable method formanufacturing the exhaust gas purification catalyst, a void formingreagent is used that has a specific shape, thermal decomposition onsettemperature, particle size distribution, and swelling degree in asolvent, and also, the amount and particle size of the void formingreagent, and the size of metal oxide particles in the slurry, and thecomposition of the metal oxides (which affects the stability of theparticle shape of the metal oxides) are appropriately tailored. The sameis also applied to the number of approximately-perfect circular voids.

In view of further improving the PM collection rate and the effect ofsuppressing pressure loss, the mass of the catalyst portions, e.g. thefirst catalyst portions 14, may be tailored according to the amount ofthe catalytically active component. In view of improving the PMcollection rate, the mass of the first catalyst portions after drying ispreferably 10 g or greater, more preferably 20 g or greater, per 1 L ofvolume of the section in the substrate where the first catalyst portions14 are formed. In view of reducing pressure loss and improving exhaustgas purification performance during high-speed operation, the mass ofthe first catalyst portions 14 after drying is preferably 80 g or less,more preferably 60 g or less, per 1 L of volume of the section in thesubstrate where the first catalyst portions 14 are formed. The mass ofthe second catalyst portions 15 may also be tailored according to theamount of the catalytically active component. In view of improving thePM collection rate, the mass of the second catalyst portions afterdrying is preferably 20 g or greater, more preferably 30 g or greater,per 1 L of volume of the section in the substrate where the secondcatalyst portions 15 are formed. In view of reducing pressure loss andimproving exhaust gas purification performance during high-speedoperation, the mass of the second catalyst portions 15 after drying ispreferably 80 g or less, more preferably 60 g or less, per 1 L of volumeof the section in the substrate where the second catalyst portions 15are formed.

The volume of a section in the substrate here refers to the apparentvolume including the substrate's partition walls 23, the first catalystportions 14, the second catalyst portions 15, the pores in the partitionwalls 23, and the spaces within the cells 21 and 22. The “volume of thesection in the substrate where the first catalyst portions 14 areformed” is the volume calculated as follows: Substrate's apparent volumex First catalyst portion 14's length L1 in direction X/Substrate 11'slength L in direction X. The “volume of the section in the substratewhere the second catalyst portions 15 are formed” is the volumecalculated as follows: Substrate's apparent volume x Second catalystportion 15's length L2 in direction X/Substrate 11's length L indirection X. (See FIG. 2 for L, L1, and L2.)

In view of further effectively balancing suppression of pressure loss,suppression of peeling and the PM collection rate, the pore volumeresulting from pores having a pore diameter from 5 to 500 nm in thecatalyst layers is preferably within a range from 0.020 to 0.20 cm³/g.For example, the mercury intrusion method in accordance with JIS R1655:2003 can be employed as the method for measuring the pore volume,and AutoPore IV9520 from Shimadzu Corporation can be used as themeasurement device.

The catalyst portions include a catalytically active component. Examplesof the catalytically active component include platinum metals. Concreteexamples include platinum (Pt), palladium (Pd), rhodium (Rh), ruthenium(Ru), iridium (Ir), and osmium (Os), and these may be used singly or incombination of two or more. In view of exhaust gas purificationperformance, it is preferable that the first catalyst layer 14 and thesecond catalyst layer 15 each independently include at least one ofmetal selected from platinum (Pt), palladium (Pd), and rhodium (Rh) asthe catalytically active component. In cases where the catalyst portionsinclude both the first catalyst layers 14 and the second catalyst layers15, the catalytically active component in the first catalyst layers 14and that in the second catalyst layers 15 may be the same or differentfrom each other. The second catalyst layers 15 preferably include, asthe catalytically active component, a catalytically active componentdifferent from that included in the first catalyst layers 14. Forexample, the following is particularly preferable in view of efficientremoval of toxic components in exhaust gas, such as NOR, CO and HC: thefirst catalyst layers 14 include a noble metal selected from platinum(Pt), palladium (Pd) and rhodium (Rh), and the second catalyst layers 15includes a noble metal selected from platinum (Pt), palladium (Pd) andrhodium (Rh), the noble metal in the second catalyst layers 15 beingdifferent from the noble metal included in the first catalyst layers 14.In view of improving NOR removal performance, it is especiallypreferable that one of “the first catalyst layers 14” and “the secondcatalyst layers 15” (particularly the first catalyst layers 14) includerhodium (Rh). Particularly, when both the first catalyst layers 14 andthe second catalyst layers 15 include rhodium (Rh) in the catalyst ofthe invention, such a catalyst of the invention will be suitable for thesecond or subsequent catalyst, which is important for NOR removal, incases where two or more catalysts are arranged in a vehicle along theexhaust gas flow direction (i.e., the second or subsequent catalyst fromthe upstream side in cases where two or more exhaust gas purificationcatalysts are arranged along the exhaust gas flow direction).

In view of exhaust gas purification performance and cost, the content ofthe catalytically active component in the catalyst portions, e.g. thefirst catalyst layers 14 and/or the second catalyst layers 15, ispreferably from 0.001 mass % to 25 mass %, more preferably from 0.01mass % to 20 mass %, even more preferably from 0.05 mass % to 15 mass %,based on the total amount of components in the first catalyst layers 14or that in the second catalyst layers 15.

The amount of the catalytically active component can be determined, forexample, by completely dissolving the catalyst layers to obtain asolution and measuring the amount of noble metals in the solution withICP-AES.

In cases where the catalyst layers are included within the substrate'spartition wall, the amount can be determined by subtracting the amountof noble metals in a solution obtained by completely dissolving only thesubstrate from the amount of noble metals in a solution obtained bycompletely dissolving the catalyst layers and the substrate.

Preferred compositions of the catalyst portions are described in furtherdetail. In view of efficiently delivering exhaust gas purificationperformance of the catalytically active component, the catalyst portionspreferably include a catalyst support component for supporting thecatalytically active component. Examples of the catalyst supportcomponent include metal oxide particles. Concrete examples of metaloxides for the metal oxide particles include inorganic oxides which areoxygen storage components (hereinbelow also called “OSC materials”), andinorganic oxides other than oxygen storage components. In the catalystportion, the catalytically active component is preferably supported onboth particles of an inorganic oxide which is an oxygen storagecomponent and particles of an inorganic oxide other than an oxygenstorage component.

Herein, “metal oxide particles” include calcined compacts in which metaloxide particles have been bonded together by calcinating.

Herein, a state in which the catalytically active component is“supported” on metal oxide particles means that the catalytically activecomponent is physically or chemically adsorbed or held on the outersurface of the metal oxide particles or on the inner surface of thepores of the metal oxide particles. Specifically, whether acatalytically active component is supported on metal oxide particles canbe determined in the following manner, for example: a cross section ofthe exhaust gas purification catalyst 10 is analyzed using EDS to obtainan elemental map, and if the presence of a metal oxide component andthat of a catalytically active component are confirmed in the sameregion of the elemental map, it is determined that the catalyticallyactive component is “supported” on the metal oxide particles.

As the inorganic oxide as an oxygen storage component, a metal oxidehaving a polyvalent state and also having the capability of storingoxygen can be used. Examples thereof include CeO₂, CZ materials(ceria-zirconia complex oxides including Ce and Zr, or solid solutionsof CeO₂ and ZrO₂), iron oxide, and copper oxide. An oxide of arare-earth element other than Ce is also preferably used in view ofthermal stability etc. Examples of the oxide of a rare-earth elementother than Ce include Sc₂O₃, Y₂O₃, La₂O₃, Pr₆O₁₁, Nd₂O₃, Sm₂O₃, Eu₂O₃,Gd₂O₃, Tb₄O₇, Dy₂O₃, Ho₂O₃, Er₂O₃, Tm₂O₃, Yb₂O₃, and LU₂O₃.

CeO₂—ZrO₂ is a solid solution of CeO₂ and ZrO₂. The formation of a solidsolution of CeO₂ and ZrO₂ can be confirmed by checking whether or not asingle phase derived from CeO₂—ZrO₂ is formed, using an X-raydiffractometer (XRD). CeO₂—ZrO₂ may be a solid solution that alsocontains the oxide of a rare earth element other than Ce.

Examples of the inorganic oxides other than oxygen storage componentsthat may be included in the catalyst portion include metal oxides otherthan oxygen storage components, and specific examples thereof includealumina, silica, silica-alumina, titania, and aluminosilicates.Particularly, alumina is preferably used in view of excellent heatresistance.

In cases where the catalyst portions include both the first catalystlayers 14 and the second catalyst layers 15, the features of thecompositions of the catalyst portions described above preferably applyto both the first catalyst layers 14 and the second catalyst portions15.

In addition to having a specific circular void percentage, the catalystportions preferably has a high content of zirconium element in terms ofoxide (amount of ZrO₂). A circular void percentage of greater than 10%and 30% or less delivers excellent PM collection performance andsuppression of pressure loss, but results in many thin portions in thecatalyst portion, thereby making the catalyst portion relativelybrittle. The poor strength of the catalyst portion decreases theadhesiveness of the catalyst portion to the substrate, which is anadverse effect of pressure loss reduction, and may result in animpractical product. As a countermeasure, the inventors have found thatincreasing the amount of ZrO₂ in the catalyst portions, which arerendered partially thin by the high circular void percentage, achieveshigh adhesiveness of the catalyst portions to the substrate. The reasonwhy an increase in the amount of ZrO₂ favorably suppresses peeling ofthe catalyst portions is probably because of characteristics of ZrO₂,including the high melting point; large bond-dissociation energy withoxygen; high flexural strength and fracture toughness, which lead todurability to mechanical action; and small change in specific surfacearea before and after calcinating. Specifically, the amount of ZrO₂ inthe catalyst portions is preferably 35 mass % or greater, morepreferably 45 mass % or greater, even more preferably 50 mass % orgreater. In view of securing a sufficient amount of CeO₂ in the catalystportions to enhance oxygen storage capacity (OSC), the content ofzirconium element in terms of oxide (amount of ZrO₂) in the catalystportions is preferably 85 mass % or less, more preferably 75 mass % orless, even more preferably 65 mass % or less.

For example, in cases where the catalyst portions include CeO₂ and ZrO₂,the mass ratio of the content of cerium element in terms of oxide(amount of CeO₂) to the content of zirconium element in terms of oxide(amount of ZrO₂) in the catalyst portions, CeO₂/ ZrO₂, is preferably 0.8or less, more preferably 0.5 or less, most preferably 0.3 or less, inview of effectively preventing peeling. In view of OSC, the ratioCeO₂/ZrO₂ is preferably 0.03 or greater.

Further, in cases where the catalyst portions include alumina, the massratio of the content of zirconium element in terms of oxide (amount ofZrO₂) to the content of aluminum element in terms of oxide (amount ofAl₂O₃), ZrO₂/Al₂O₃, is preferably 1.5 or greater, more preferably 2.5 orgreater, even more preferably 3.5 or greater. In view of OSC, the ratioZrO₂/Al₂O₃ is preferably 10 or less.

Further, in cases where the catalyst portions include alumina and ceria,the mass ratio of the total of the content of zirconium element in termsof oxide (amount of ZrO₂) and the content of cerium element in terms ofoxide (amount of CeO₂) to the content of aluminum element in terms ofoxide (amount of Al₂O₃) in the catalyst portions, (ZrO₂+CeO₂)/Al₂O₃, ispreferably 3.0 or greater, more preferably 3.5 or greater, even morepreferably 4.0 or greater. In view of OSC, the ratio (ZrO₂+CeO₂)/Al₂O₃is preferably 10 or less.

Typically, the shape of ZrO₂ particles tends to be stable, whereas theshape of Al₂O₃ particles tends to be unstable. Thus, generally speaking,a composition having a large value of ZrO₂/Al₂O₃ tends to have a highcircular void percentage compared to a composition having a small valueof ZrO₂/Al₂O₃.

In view of balancing heat resistance and OSC, the content of ceriumelement in terms of oxide (amount of CeO₂) in the catalyst portions ispreferably from 5 mass % to 40 mass %, more preferably from 10 mass % to30 mass %.

Herein, “the content of cerium element in terms of oxide (amount ofCeO₂)” and “the content of zirconium element in terms of oxide (amountof ZrO₂)” in the catalyst portions include the amounts of CeO₂ and ZrO₂in solid-solution form, respectively, and also include the amount of Cein terms of CeO₂ and the amount of Zr in terms of ZrO₂ in ceria-zirconiacomplex oxides, respectively. For example, the amount of CeO₂ and theamount of ZrO₂ can be determined by completely dissolving the catalystlayers to obtain a solution, measuring the amounts of Ce and Zr in thesolution with ICP-AES, and converting the found amounts into amounts ofrespective oxides.

In cases where the catalyst layers are included within the substrate'spartition wall, the amount can be determined by subtracting the amountsof Ce and Zr in a solution obtained by completely dissolving only thesubstrate from the amounts of Ce and Zr in a solution obtained bycompletely dissolving the catalyst layers and the substrate.

In view of balancing heat resistance, OSC and suppression of pressureloss, the content of the inorganic oxides other than the oxygen storagecomponent in the catalyst portions is preferably from 4 mass % to 50mass %, more preferably from 7 mass % to 30 mass %. For example, thecontent of aluminum element in terms of oxide (amount of Al₂O₃) can bedetermined by completely dissolving the catalyst layers to obtain asolution, measuring the amount of aluminum in the solution with ICP-AES,and converting the found amount into the amount of oxide.

In cases where the catalyst layers are included within the substrate'spartition wall, the amount can be determined by subtracting the amountof Al in a solution obtained by completely dissolving only the substratefrom the amount of Al in a solution obtained by completely dissolvingthe catalyst layers and the substrate.

In cases where the catalyst portions include the first catalyst layers14 and the second catalyst layers 15, the above-described features ofthe amount of ZrO₂, the amount of CeO₂ and/or the amount of Al₂O₃ mayapply only to either one of “the first catalyst layers 14” or “thesecond catalyst layers 15” of the catalyst portions, or may apply toboth. Preferably, both the first catalyst layers 14 and the secondcatalyst layers 15 have the following features.

The features of the exhaust gas purification catalyst 10 are describedin further detail.

In view of further improving PM collection performance and improvingexhaust gas purification performance during high-speed operation, thefirst catalyst layer 14 is preferably present mainly on the surface ofthe partition wall 23 and not within the partition wall 23. “The firstcatalyst layer 14 is present mainly on the surface of the partition wall23” means that, in a cross section of the substrate 11 on which thefirst catalyst layers 14 are provided, the mass of the first catalystlayer 14 present on the surface of the substrate 11's partition wall 23is greater than the mass of the first catalyst layer 14 present withinthe partition wall 23. Whether or not a catalyst layer is mainly presenton the surface can be checked in the following manner: a cross sectionof the partition wall on which the first catalyst layer 14 is providedis observed with a scanning electron microscope (JEM-ARM200F from JEOLLtd.) and also analyzed with energy dispersive X-ray spectrometry (EDS),to thereby line-analyze the boundary between the distribution ofelements (for example, Si, and Mg) that exist only in the substrate andthe distribution of elements (for example, Ce and Zr) that exist only inthe catalyst layer. Alternatively, that can also be checked by analyzingthe cross section with an electron probe micro analyzer (EPMA). Also,the second catalyst layer 15 is preferably present mainly on the surfaceof the partition wall 23 and not within the partition wall 23.

In view of delivering suitable PM collection performance while reducingpressure loss, the length L1 (see FIG. 2 ), in the direction X, of thefirst catalyst layer 14 is preferably from 10 to 80%, more preferablyfrom 30 to 60%, based on the length L (see FIG. 2 ), in the direction X,of the substrate 11. In view of improving PM collection performancewhile reducing pressure loss, the length L2 (see FIG. 2 ), in thedirection X, of the second catalyst layer 15 is preferably from 30 to90%, more preferably from 50 to 80%, based on the length L, in thedirection X, of the substrate 11. It is preferable to form the firstcatalyst layer 14 from the upstream end in the exhaust gas flowdirection, and to form the second catalyst layer 15 from the downstreamend in the exhaust gas flow direction.

In view of improving exhaust gas purification performance, the totallength of the first catalyst layer 14's length L1 in the direction X andthe second catalyst layer 15's length L2 in the direction X, L1+L2, ispreferably longer than the substrate 11's length L in the direction X;and (L1+L2)/L is preferably 1.05 or greater, more preferably 1.10 orgreater.

The length of the first catalyst layer 14 and the second catalyst layer15 can be measured according to the following method. Specifically, itis preferable to visually observe the exhaust gas purification catalyst10 to determine the boundary of the first catalyst layer 14 and theboundary of the second catalyst layer 15, and then measure the length ofeach of the first catalyst layer 14 and the second catalyst layer 15. Atthis time, it is preferable to measure the length of each of the firstcatalyst layer 14 and the second catalyst layer 15 at, for example, tendiscretionary sites in the exhaust gas purification catalyst 10, anddetermine the respective average values as the length of the firstcatalyst layer 14 and that of the second catalyst layer 15. If it isimpossible to visually determine the boundary, in the exhaust gas flowdirection, of the first catalyst layer 14, the second catalyst layer 15,the lower layer 15A and/or the upper layer 15B, the boundary can beidentified in the following manner: the composition is analyzed atmultiple positions (e.g., 8 to 16 positions) on the exhaust gaspurification catalyst along the exhaust gas flow direction, and theboundary is identified on the basis of the content of a catalyticallyactive component in the composition at each position. The content of acatalytically active component at each position can be determined by,for example, X-ray fluorescence analysis (XRF) or ICP emissionspectrometry (ICP-AES).

In view of achieving both ease of manufacturing and exhaust gaspurification performance, it is preferable to form the first catalystlayers 14 so as to extend from the substrate 11's upstream end portionR1 toward the downstream side in the direction X. It is also preferableto form the second catalyst layers 15 so as to extend from the substrate11's downstream end portion R2 toward the upstream side in the directionX. Further, in cases where the second catalyst layers 15 each include alower layer 15A and an upper layer 15B as described below, it ispreferable to form the lower layer 15A and the upper layer 15B so as toextend from the substrate 11's downstream end portion R2 toward theupstream side in the direction X.

Next, a preferable method for manufacturing an exhaust gas purificationcatalyst according to the present invention is described below.

In this manufacturing method, catalyst portions are formed in asubstrate, and the substrate includes: inflow-side cells, eachinflow-side cell being a space having an open end on an inflow sidethereof and a closed end on an outflow side thereof in an exhaust gasflow direction; outflow-side cells, each outflow-side cell being a spacehaving a closed end on an inflow side thereof and an open end on anoutflow side thereof in the exhaust gas flow direction; and porouspartition walls, each porous partition wall separating the inflow-sidecell and the outflow-side cell from each other. More specifically,first, a slurry for forming catalyst portions, the slurry including avoid forming reagent, is applied to a surface of the partition wallsfacing the inflow-side cell and/or a surface of the partition wallsfacing the outflow-side cell, and then, the slurry applied to thesubstrate is calcinated to eliminate the void forming reagent, tothereby form the catalyst portions including a plurality of voids. Inview of satisfying the circular void percentage within theaforementioned range, at least 90% of the particles of the void formingreagent preferably has a roundness of L/{2(πS)^(1/2)}≤1.05, and thethermal decomposition onset temperature of the void forming reagent ispreferably 200° C. or higher. Further, the content of zirconium elementin terms of oxide (amount of ZrO₂) is preferably from 35 mass % to 85mass % based on the solid contents excluding the void forming reagent inthe slurry.

In the void forming reagent added to the slurry for forming catalystportions in the present manufacturing method, at least 90% of theparticles of the void forming reagent, in terms of the number ofparticles, preferably has a roundness of L/{2(πS)^(1/2)}≤1.05. Thisenables an exhaust gas purification catalyst 10 having theaforementioned circular void percentage to be obtained easily. Herein,the “roundness” is measured through observation with a SEM. A sample tobe observed can be obtained by: bonding a carbon tape onto a SEM samplestage; dropping a powder sample attached to a cotton bud, from aboveonto the stage; and then blowing away excessive powder with an air gun.For example, air may be blown with the air gun from a position separatedby 10 cm at 5 atmospheres (gauge pressure) for 1 second, though thecondition is not limited thereto.

SEM observation is preferably performed at an acceleration voltage of 5to 15 kV under an observation magnification of 40× to 1000×. SEM imagesof discretionarily selected 50 particles of the void forming reagent areobserved, to determine the percentage of the void forming reagentparticles whose contour in the SEM image has a roundness ofL/{2(πS)^(1/2)}≤1.05.

In view of achieving the aforementioned circular void percentage moreeasily, the percentage of the void forming reagent particles having aroundness of L/{2(πS)^(1/2)}≤1.05 is more preferably 95% or greater,even more preferably 98% or greater.

The void forming reagent used in the present manufacturing methodpreferably has a thermal decomposition onset temperature of 200° C. orhigher in the atmosphere. The void forming reagent with a high thermaldecomposition temperature typically has a high degree of crosslinking ofmolecular chains. This prevents the shape of the void forming reagentfrom getting deformed during calcinating, thereby allowing voids withhigh roundness to be left in the catalyst portion, and thus facilitatingmanufacturing of an exhaust gas purification catalyst 10 having theaforementioned circular void percentage. The thermal decomposition onsettemperature of the void forming reagent in the atmosphere is morepreferably 250° C. or higher, even more preferably 270° C. or higher. Inview of making the void forming reagent disappear reliably duringcalcinating, the thermal decomposition onset temperature is preferably550° C. or lower, more preferably 500° C. or lower. As regards thethermal decomposition temperature, for example, the thermaldecomposition behavior is analyzed in the atmosphere by raising thetemperature from room temperature to 500° C. The rate of temperaturerise may be from 5 to 20° C./minute, for example. The thermaldecomposition onset temperature is determined from the intersectionpoint between: a line parallel to the horizontal axis and passingthrough the mass before starting of heating in the test; and a tangentdrawn so as to maximize the gradient between inflection points in thedecomposition curve. In Examples described later, the thermaldecomposition onset temperature in the atmosphere is simply described as“thermal decomposition onset temperature”.

In the particle size distribution of the void forming reagent asmeasured according to the laser diffraction particle size distributionmethod, the value of (D90-D10)/D50 is preferably from 0.1 to 1.1,wherein the particle size at a cumulative volume of 10% is designated asD10, the particle size at a cumulative volume of 50% is designated asD50, and the particle size at a cumulative volume of 90% is designatedas D90. The value of (D90-D10)/D50 (hereinbelow also called“monodispersity”) is a measure indicating the particle size distributionof the void forming reagent; the smaller the value of themonodispersity, the sharper the particle size distribution. The voidforming reagent with a sharp particle size distribution has a highdegree of crosslinking, and are thus excellent in terms of furthersuppressing deformation of the void forming reagent during calcinatingdue to thermal expansion, thereby allowing the aforementioned circularvoid percentage to be achieved more easily. From this viewpoint, themonodispersity of the void forming reagent is preferably 0.7 or less,more preferably 0.3 or less. In view of obtaining voids having theaforementioned circle equivalent diameter, D50 of the void formingreagent is preferably from 5 to 50 μm, more preferably from 10 to 30 μm.In view of monodispersity, D90 of the void forming reagent is preferablyfrom 8 to 60 μm, more preferably from 15 to 40 μm. From the sameviewpoint, D10 is preferably from 2 to 40 μm, more preferably from 5 to20 μm. The monodispersity and particle size can be determined using, forexample, a laser diffraction/scattering particle size/particle sizedistribution analyzer, such as Microtrac HRA or the Microtrac 3000series from MicrotracBEL Corp. For example, analysis can be performed asfollows. An automatic sample feeder (Microtorac SDC from MicrotracBELCorp.) for a laser diffraction particle size distribution analyzer isused; void forming reagent particles are added to an aqueous dispersionmedium, followed by application of ultrasonic waves at 40 W to theresulting mixture at a flow rate of 40% for 360 seconds. Then,measurement is performed using a laser diffraction/scattering methodparticle size distribution analyzer (Microtrac MT3300 EXII fromMicrotracBEL Corp.). The measurement conditions are as follows: particlerefractive index: 1.5; particle shape: perfect sphere; solventrefractive index: 1.3; set zero: 30 seconds; measurement time: 30seconds. Measurement is performed twice, and the average value iscalculated. Pure water is used as the aqueous dispersion medium.

It is also preferable that the void forming reagent used in the presentmanufacturing method have a low swelling degree in a solvent. This isbecause a void forming reagent with a low swelling degree has a highdegree of crosslinking. For example, the amount (g) of solvent absorbedper 1 g of polymer after immersion at 50° C. for 48 hours is preferably0.05 g/g or less in ethanol, preferably 0.7 g/g or less in acetone,preferably 0.15 g/g or less in 2-methoxyethanol, and preferably 0.15 g/gor less in toluene.

Examples of materials for the void forming reagent include polymers ofmonomers having an ethylenically unsaturated bond includingcrosslinkable monomers, and crosslinked acrylic resin and crosslinkedstyrene resin are usable, for example. Particularly, crosslinkedpolystyrene particles or crosslinked poly(meth)acrylic ester particlescan be used, for example. Examples of crosslinked poly(meth)acrylicester particles include crosslinked poly(methyl (meth)acrylate)particles and crosslinked poly(butyl (meth)acrylate) particles.

The amount of the void forming reagent is preferably greater than 15mass % to 40 mass % or less, more preferably greater than 20 mass % to35 mass % or less relative to the solid contents excluding the voidforming reagent in the slurry for forming catalyst portions.

Further, the slurry including the void forming reagent preferablyincludes a catalytically active component and metal oxide particles forsupporting the catalytically active component. Examples of the metaloxide particles include particles of an inorganic oxide which is anoxygen storage component and particles of an inorganic oxide other thanan oxygen storage component, as described above as constituentcomponents of the catalyst portion.

In view of improving the dispersibility of the catalytically activecomponent and improving PM collection performance, D50 of the particlesize of the metal oxide particles in the catalyst portion is preferably1 μm or greater, more preferably 2 μm or greater, and D90 thereof ispreferably 7 μm or greater, more preferably 15 μm or greater. In view ofimproving the dispersibility of the catalytically active component, D50of the particle size of the metal oxide particles in the slurry ispreferably 40 μm or less, and D90 thereof is preferably 80 μm or less.

D50 and D90 of the metal oxide particles may be the particle size in astate where the metal oxide particles support the catalytically activecomponent, or the particle size before the metal oxide particles supportthe catalytically active component. D50 and D90 may be equal to or abovethe aforementioned preferable lower limit, or equal to or below theaforementioned preferable upper limit, either before or after supportingthe catalytically active component. D50 and D90 of the metal oxideparticles can be determined according to the same method as that for D50and D90 of the void forming reagent.

The slurry can be obtained by mixing the catalytically active componentin the form of a water-soluble salt, such as a nitrate, with the metaloxide particles, and the slurry can be applied to the substrate 11 andthen dried or calcinated. Alternatively, the catalytically activecomponent can be supported by the metal oxide particles in advance, andthen the catalytically active component supported on the metal oxideparticles can be made into a slurry.

In the slurry, the composition of the solid contents excluding the voidforming reagent is preferably the same as the preferable compositiondescribed above for the catalyst portions of the exhaust gaspurification catalyst 10. Therefore, the content of zirconium element interms of oxide (amount of ZrO₂) in the slurry is preferably from 35 to85 parts by mass based on 100 parts by mass of components of the solidcontents excluding the void forming reagent.

In the first step, the slurry including the aforementioned constituentcomponents is applied to the surfaces of the partition walls that eachface the inflow-side cell and/or the surfaces of the partition wallsthat each face the outflow-side cell. In order to apply the slurry tothe partition wall's surfaces that each face the inflow-side cell, thesubstrate 11's upstream side in the exhaust gas flow direction may beimmersed in the slurry, for example. In order to apply the slurry to thepartition wall's surfaces that each face the outflow-side cell, thesubstrate 11's downstream side in the exhaust gas flow direction may beimmersed in the slurry, for example. The slurry may be sucked from theother side simultaneously with the immersion.

In the second step, the slurry applied to the substrate is calcinated,to eliminate the void forming reagent and form the catalyst portionsincluding a plurality of voids. In view of preventing deterioration incatalyst activity and successfully calcinating successfully calcinatingand eliminating the void forming reagent, the calcinating temperature ispreferably from 350 to 550° C. In cases of drying the slurry beforecalcinating, the drying temperature is preferably from 40 to 120° C.Calcinating is typically performed in the atmosphere.

The exhaust gas purification catalyst 10 manufactured as above can beused for various applications as an exhaust gas purification catalystfor internal combustion engines that employ fossil fuel as a source ofpower, such as gasoline engines, by taking advantage of PM collectionperformance and pressure loss prevention performance as well as peelingprevention performance. Further, according to the present embodiment,there can also be provided an exhaust gas purification method involvinguse of the exhaust gas purification catalyst 10. For example, exhaustgas from the gasoline engine can be purified favorably when the exhaustgas purification catalyst 10 is set in an exhaust path of an internalcombustion engine such as a gasoline engine, and particularly a GDIengine of a vehicle, and is used as a GPF etc. Particularly, in caseswhere two or more exhaust gas purification catalysts are arranged alongthe exhaust gas flow direction in an exhaust gas purification device,the exhaust gas purification catalyst 10 is preferably used as thesecond or subsequent catalyst from the upstream side. The reason forthis is as follows: because the nature of filter catalysts constrainsthe amount of the slurry applied, use of the present exhaust gaspurification catalyst as the first catalyst, which is subjected to highthermal load, will promote degradation, whereas use of the presentexhaust gas purification catalyst as the second or subsequent catalystcan favorably maintain high purification performance and PM collectionperformance.

EXAMPLES

The present invention is described in further detail below by way ofexamples. The scope of the present invention, however, is not limited tothe examples.

Drying and calcinating were all performed in the atmosphere. The thermaldecomposition onset temperature in each example was measured in theatmosphere. In the following examples, the amount of a void formingreagent relative to solid contents all means the amount relative tosolid contents excluding the void forming reagent.

Example 1

1. Preparation of First Slurry:

CeO₂—ZrO₂ solid solution powder (the CeO₂—ZrO₂ solid solution contained15 mass % of CeO₂, 70 mass % of ZrO₂, and 15 mass % of an oxide of arare-earth element other than Ce) and alumina powder were provided. TheCeO₂—ZrO₂ solid solution powder and the alumina powder were mixed, andimmersed in a rhodium nitrate aqueous solution.

Next, with this liquid mixture, a spherical void forming reagent(crosslinked poly(methyl (meth)acrylate) particles; D50: 20 μm; D90: 22μm; D10: 18 μm; monodispersity: 0.165; thermal decomposition onsettemperature: 250° C.; roundness: 95%; amount (g) of solvent absorbed per1 g of polymer after immersion at 50° C. for 48 hours: 0.02 g/g inethanol, 0.24 g/g in acetone, 0.05 g/g in 2-methoxyethanol, and 0 g/g intoluene); alumina sol; zirconia sol; and water as a liquid solvent weremixed to prepare a first slurry. Here, the “roundness” is thepercentage, in terms of number, of particles satisfyingL/{2(πS)^(1/2)}≤1.05 among the particles of the spherical void formingreagent. The amount of the CeO₂—ZrO₂ solid solution powder was 80 partsby mass, the amount of the alumina powder was 11 parts by mass, theamount of the alumina sol was 3.0 parts by mass, the amount of thezirconia sol was 5.0 parts by mass, and the amount of Rh was 1.0 part bymass in terms of metal, all based on 100 parts by mass of the components(excluding the void forming reagent) of the solid contents of theslurry. The mass percentage of the void forming reagent was 20% relativeto the solid contents of the slurry. D50 of the metal oxides in theslurry was 8 μm, and D90 thereof was 22 μm. SEM observation formeasuring the roundness of the void forming reagent as a raw materialwas performed at an acceleration voltage of 15 kV and an observationmagnification of 600×.

2. Forming Precursor of First Catalyst Portion:

A substrate 11 having the structure illustrated in FIG. 1 was used. Thesubstrate 11 had a volume of 1.0 L and an entire length of 91 mm, andincluded 300 cells/inch in a plane vertical to the axial direction, eachcell extending in the axial direction and being defined by partitionwalls each having a thickness of 200 to 250 μm. In the substrate 11, thearea of the opening of a single inflow-side cell 21 in the end face onthe inflow-side was substantially the same as the area of the opening ofa single outflow-side cell 22 in the end face on the outflow-side.

The upstream-side end portion of the substrate 11 in the exhaust gasflow direction was immersed in the first slurry, and the slurry wasdrawn by suction from the downstream side. Then the slurry was dried at70° C. for 10 minutes. In this way, a layer made of the solid contentsof the first slurry (i.e., a precursor of the first catalyst portion)was formed on each partition wall 23's surface that faced theinflow-side cell 21.

3. Forming Precursor of Second Catalyst Portion:

The downstream-side end portion of the substrate 11 in the exhaust gasflow direction after drying was immersed in the first slurry, and theslurry was drawn by suction from the upstream side. Then the slurry wasdried at 70° C. for 10 minutes. In this way, a layer made of the solidcontents of the first slurry (i.e., a precursor of the second catalystportion) was formed on each partition wall 23's surface that faced theoutflow-side cell 22.

4. Calcinating:

The substrate 11 was then calcinated at 450° C. for 1 hour, whereby anexhaust gas purification catalyst 10 of Example 1 having first catalystportions 14 and second catalyst portions 15 formed in the substrate 11was obtained.

In the exhaust gas purification catalyst of Example 1, the firstcatalyst portion 14 was formed on each partition wall 23's surface onthe inflow-side cell 21-side so as to extend from the upstream-side endportion R1 toward the downstream side in the exhaust gas flow directionX by a length of 45% based on the entire length L. The mass of the firstcatalyst portions 14 after calcinating per volume of the section in thesubstrate where the first catalyst portions 14 were formed was 52 g/L.The second catalyst portion 15 of the exhaust gas purification catalyst10 was formed on each partition wall 23's surface on the outflow-sidecell 22-side so as to extend from the downstream-side end portion R2toward the upstream side in the exhaust gas flow direction X by a lengthof 70% based on the entire length L. The mass of the second catalystportions 15 after calcinating per volume of the section in the substratewhere the second catalyst portions 15 were formed was 52 g/L.

Example 2

An exhaust gas purification catalyst 10 was obtained as in Example 1,except that the mass percentage of the void forming reagent in the firstslurry was changed to 30% relative to the solid contents of the slurry.

Example 3

An exhaust gas purification catalyst 10 was obtained as in Example 1,except that the mass percentage of the void forming reagent in the firstslurry was changed to 40% relative to the solid contents of the slurry.

Example 4

An exhaust gas purification catalyst 10 was obtained as in Example 1,except that the void forming reagent in Example 1 was replaced with thatdescribed below, and that the mass percentage of the void formingreagent was changed to 50% relative to the solid contents of the slurry.The void forming reagent used in Example 4 was a spherical void formingreagent having D50 of 5 μm (material: crosslinked poly(methyl(meth)acrylate) particles; D90: 7μm; D10: 3 μm; monodispersity: 0.120;thermal decomposition onset temperature: 250° C.; roundness: 95%; amount(g) of solvent absorbed per 1 g of polymer after immersion at 50° C. for48 hours: 0.03 g/g in ethanol, 0.43 g/g in acetone, 0.08 g/g in2-methoxyethanol, and 0.02 g/g in toluene).

Example 5

An exhaust gas purification catalyst 10 was obtained as in Example 1,except that the void forming reagent in Example 1 was replaced with aspherical void forming reagent having D50 of 50 μm (material:crosslinked poly(methyl (meth)acrylate) particles; D90: 80 μm; D10: 28μm; monodispersity: 1.039; thermal decomposition onset temperature: 250°C.; roundness: 90%; amount (g) of solvent absorbed per 1 g of polymerafter immersion at 50° C. for 48 hours: 0.04 g/g in ethanol, 0.66 g/g inacetone, 0.12 g/g in 2-methoxyethanol, and 0.12 g/g in toluene).

Example 6

An exhaust gas purification catalyst 10 was obtained as in Example 1,except that preparation of the first slurry in Example 1 was modified asfollows.

Modification of Preparation of Slurry:

Zirconia powder and alumina powder were provided. The zirconia powderand the alumina powder were mixed, and immersed in a rhodium nitrateaqueous solution.

Next, with this liquid mixture, a spherical void forming reagent(crosslinked poly(methyl (meth)acrylate) particles; D50: 20 p.m; D90: 22p.m; D10: 18 p.m; monodispersity: 0.165; thermal decomposition onsettemperature: 250° C.; roundness: 95%; amount (g) of solvent absorbed per1 g of polymer after immersion at 50° C. for 48 hours: 0.02 g/g inethanol, 0.24 g/g in acetone, 0.05 g/g in 2-methoxyethanol, and 0 g/g intoluene); alumina sol; zirconia sol; and water as a liquid solvent weremixed to prepare a slurry. The amount of the zirconia powder was 80parts by mass, the amount of the alumina powder was 11 parts by mass,the amount of the alumina sol was 3.0 parts by mass, the amount of thezirconia sol was 5.0 parts by mass, and the amount of Rh was 1.0 part bymass in terms of metal, all based on 100 parts by mass of the components(excluding the void forming reagent) of the solid contents of theslurry. D50 of the metal oxides in the slurry was 6 μm, and D90 thereofwas 20 μm. The mass percentage of the void forming reagent was 15%relative to the solid contents of the slurry.

Example 7

An exhaust gas purification catalyst 10 was obtained as in Example 1,except that preparation of the first slurry in Example 1 was modified asfollows, and that the amount of the catalyst portions was changed.

Modification of Preparation of Slurry:

Zirconia powder and alumina powder were provided. The zirconia powderand the alumina powder were mixed, and immersed in an aqueous solutionof rhodium nitrate and palladium nitrate.

Next, with this liquid mixture, a spherical void forming reagent(crosslinked poly(methyl (meth)acrylate) particles; D50: 20 μm; D90: 22μm; D10: 18 μm; monodispersity: 0.165; thermal decomposition onsettemperature: 250° C.; roundness: 95%; amount of solvent absorbed (g) per1 g of polymer after immersion at 50° C. for 48 hours: 0.02 g/g inethanol, 0.24 g/g in acetone, 0.05 g/g in 2-methoxyethanol, and 0 g/g intoluene); barium hydroxide; alumina sol; zirconia sol; ceria sol; andwater as a liquid solvent were mixed to prepare a slurry. The amount ofthe zirconia powder was 75 parts by mass, the amount of the aluminapowder was 8.7 parts by mass, the amount of barium hydroxide was 5.9parts by mass, the amount of the alumina sol was 3.0 parts by mass, theamount of the zirconia sol was 3.7 parts by mass, the amount of theceria sol was 2.0 parts by mass, the amount of Rh was 0.4 parts by massin terms of metal, and the amount of Pd was 1.3 parts by mass in termsof metal, all based on 100 parts by mass of the components (excludingthe void forming reagent) of the solid contents of the slurry. D50 ofthe metal oxides in the slurry was 10 μm, and D90 thereof was 25 μm. Themass percentage of the void forming reagent was 30% relative to thesolid contents of the slurry. The mass of the first catalyst portionsafter calcinating per volume of the substrate in the section where thefirst catalyst portions were formed was 39 g/L. The mass of the secondcatalyst portions after calcinating per volume of the substrate in thesection where the second catalyst portions were formed was 39 g/L.

Example 8

An exhaust gas purification catalyst 10 was obtained as in Example 1,except that preparation of the first slurry in Example 1 was modified asfollows, and that the amount of the catalyst portions was changed.

Modification of Preparation of Slurry:

CeO₂—ZrO₂ solid solution powder A (the CeO₂—ZrO₂ solid solutioncontained 42 mass % of CeO_(2, 46) mass % of ZrO₂, and 12 mass % of anoxide of a rare-earth element other than Ce), CeO₂—ZrO₂ solid solutionpowder B having a different composition from that of powder A (theCeO₂—ZrO₂ solid solution contained 15 mass % of CeO_(2, 70) mass % ofZrO₂, and 15 mass % of an oxide of a rare-earth element other than Ce),and alumina powder were provided. The CeO₂—ZrO₂ solid solution powdersand the alumina powder were mixed, and immersed in an aqueous solutionof rhodium nitrate and palladium nitrate.

Next, with this liquid mixture, a spherical void forming reagent(crosslinked poly(methyl (meth)acrylate) particles; D50: 20 μm; D90: 22μm; D10: 18 μm; monodispersity: 0.165; thermal decomposition onsettemperature: 250° C.; roundness: 95%; amount (g) of solvent absorbed per1 g of polymer after immersion at 50° C. for 48 hours: 0.02 g/g inethanol, 0.24 g/g in acetone, 0.05 g/g in 2-methoxyethanol, and 0 g/g intoluene); barium hydroxide; alumina sol; zirconia sol; ceria sol; andwater as a liquid solvent were mixed to prepare a slurry. The amount ofthe CeO₂—ZrO₂ solid solution powder A was 40 parts by mass, the amountof the CeO₂—ZrO₂ solid solution powder B was 27 parts by mass, theamount of the alumina powder was 17 parts by mass, the amount of bariumhydroxide was 5.6 parts by mass, the amount of the alumina sol was 3parts by mass, the amount of the zirconia sol was 3.7 parts by mass, theamount of the ceria sol was 2.0 parts by mass, the amount of Rh was 0.4parts by mass in terms of metal, and the amount of Pd was 1.3 parts bymass in terms of metal, all based on 100 parts by mass of the components(excluding the void forming reagent) of the solid contents of theslurry. D50 of the metal oxides in the slurry was 20 μm, and D90 thereofwas 35 μm. The mass percentage of the void forming reagent was 30%relative to the solid contents of the slurry. The mass of the firstcatalyst portions after calcinating per volume of the substrate in thesection where the first catalyst portions were formed was 39 g/L. Themass of the second catalyst portions after calcinating per volume of thesubstrate in the section where the second catalyst portions were formedwas 39 g/L.

Example 9

An exhaust gas purification catalyst 10 was obtained as in Example 1,except that preparation of the first slurry in Example 1 was modified asfollows.

Modification of Preparation of Slurry:

Zirconia powder was provided. The zirconia powder was immersed in arhodium nitrate aqueous solution.

Next, with this liquid mixture, a spherical void forming reagent(crosslinked poly(methyl (meth)acrylate) particles; D50: 20 μm; D90: 22μm; D10: 18 μm; monodispersity: 0.165; thermal decomposition onsettemperature: 250° C.; roundness: 95%; amount (g) of solvent absorbed per1 g of polymer after immersion at 50° C. for 48 hours: 0.02 g/g inethanol, 0.24 g/g in acetone, 0.05 g/g in 2-methoxyethanol, and 0 g/g intoluene); alumina sol; zirconia sol; and water as a liquid solvent weremixed to prepare a slurry. The amount of the zirconia powder was 92.5parts by mass, the amount of the alumina sol was 1.5 parts by mass, theamount of the zirconia sol was 5.0 parts by mass, and the amount of Rhwas 1.0 part by mass in terms of metal, all based on 100 parts by massof the components (excluding the void forming reagent) of the solidcontents of the slurry. D50 of the metal oxides in the slurry was 7 μm,and D90 thereof was 20 μm.

The mass percentage of the void forming reagent was 20% relative to thesolid contents of the slurry.

Comparative Example 1

An exhaust gas purification catalyst 10 was obtained as in Example 1,except that the amount of the void forming reagent in the first slurrywas changed to 50 mass % relative to the solid contents of the slurry.

Comparative Example 2

An exhaust gas purification catalyst was obtained as in Example 1,except that no void forming reagent was used in the preparation of theslurry.

Comparative Example 3

An exhaust gas purification catalyst was obtained as in Example 1,except that the amount of the void forming reagent in the first slurrywas changed to 10 mass % relative to the solid contents of the slurry.

Comparative Example 4

An exhaust gas purification catalyst 10 was obtained as in Example 1,except that preparation of the first slurry in Example 1 was modified asfollows.

Modification of Preparation of Slurry:

CeO₂—ZrO₂ solid solution powder (the CeO₂—ZrO₂ solid solution contained40 mass % of CeO₂, 50 mass % of ZrO₂, and 10 mass % of an oxide of arare-earth element other than Ce) and alumina powder were provided. TheCeO₂—ZrO₂ solid solution powder and the alumina powder were mixed, andimmersed in a rhodium nitrate aqueous solution.

Next, with this liquid mixture, a spherical void forming reagent(crosslinked poly(methyl (meth)acrylate) particles; D50: 20 μm; D90: 22μm; D10: 18 μm; monodispersity: 0.165; thermal decomposition onsettemperature: 250° C.; roundness: 95%; amount (g) of solvent absorbed per1 g of polymer after immersion at 50° C. for 48 hours: 0.02 g/g inethanol, 0.24 g/g in acetone, 0.05 g/g in 2-methoxyethanol, and 0 g/g intoluene); barium hydroxide; alumina sol; zirconia sol; ceria sol; andwater as a liquid solvent were mixed to prepare a slurry. The amount ofthe CeO₂—ZrO₂ solid solution powder was 59 parts by mass, the amount ofthe alumina powder was 22 parts by mass, the amount of barium hydroxidewas 9.0 parts by mass, the amount of the alumina sol was 3 parts bymass, the amount of the zirconia sol was 3.0 parts by mass, the amountof the ceria sol was 3.0 parts by mass, and the amount of Rh was 1.0parts by mass in terms of metal, all based on 100 parts by mass of thecomponents (excluding the void forming reagent) of the solid contents ofthe slurry. The mass percentage of the void forming reagent was 50%relative to the solid contents of the slurry.

Comparative Example 5

An exhaust gas purification catalyst was obtained as in ComparativeExample 4, except that the void forming reagent used in ComparativeExample 4 were replaced with the void forming reagent used in Example 4,and that the amount thereof used was changed to 25 mass % relative tothe solid contents of the slurry.

Comparative Example 6

An exhaust gas purification catalyst was obtained as in ComparativeExample 4, except that the void forming reagent used in ComparativeExample 4 for the first slurry was replaced with acicular particles(material: cellulose) having an average diameter of 10 μm, and that theamount thereof used was 25 mass % relative to the solid contents of theslurry.

Comparative Example 7

An exhaust gas purification catalyst was obtained as in ComparativeExample 4, except that the void forming reagent used in ComparativeExample 4 for the first slurry was replaced with acicular particles(material: cellulose) having an average diameter of 28 μm, and that theamount thereof used was 25 mass % relative to the solid contents of theslurry.

Comparative Example 8

An exhaust gas purification catalyst 10 was obtained as in Example 1,except that preparation of the first slurry in Example lwas modified asfollows.

Modification of Preparation of Slurry:

Zirconia powder and alumina powder were provided. The zirconia powderand the alumina powder were mixed, and immersed in a rhodium nitrateaqueous solution.

Next, with this liquid mixture, a spherical void forming reagent(crosslinked poly(methyl (meth)acrylate) particles; D50: 20 μm; D90: 22μm; D10: 18 μm; monodispersity: 0.165; thermal decomposition onsettemperature: 250° C.; roundness: 95%; amount (g) of solvent absorbed per1 g of polymer after immersion at 50° C. for 48 hours: 0.02 g/g inethanol, 0.24 g/g in acetone, 0.05 g/g in 2-methoxyethanol, and 0 g/g intoluene); alumina sol; zirconia sol; and water as a liquid solvent weremixed to prepare a slurry. The amount of the zirconia powder was 85parts by mass, the amount of the alumina powder was 6.0 parts by mass,the amount of the alumina sol was 3.0 parts by mass, the amount of thezirconia sol was 5.0 parts by mass, and the amount of Rh was 1.0 part bymass in terms of metal, all based on 100 parts by mass of the components(excluding the void forming reagent) of the solid contents of theslurry. The mass percentage of the void forming reagent was 20% relativeto the solid contents of the slurry.

Circular Void Percentage, Circle Equivalent Diameter ofApproximately-Perfect Circular Void, Number of Approximately-PerfectCircular Voids per 1 mm², and Ratio of Thickness of Catalyst Portion toThickness of Partition Wall:

For each of the exhaust gas purification catalysts obtained in Examples1 to 9 and Comparative Examples 1 to 8, the various parameters shown inTable 1, including the circular void percentage, were determinedaccording to the aforementioned methods in which the width of thedivision was equal to the median diameter of the circle equivalentdiameters of approximately-perfect circular voids in 20 fields-of-view.The sampling method was as described hereinbefore. The observationsurfaces in each sample were positioned 10 mm away from theupstream-side end face or downstream-side end face in the exhaust gasflow direction X, and a total of 20 fields-of-view were observed,including 10 fields-of-view on the observation surface 10 mm away fromthe upstream-side end face and 10 fields-of-view on the observationsurface 10 mm away from the downstream-side end face. As illustrated inFIG. 3(c), each sample was a section having a thickness of 10 mm fromthe observation surface toward the inner side in the direction X. Theobservation magnification for EPMA mapping was 300×, and theacceleration voltage was either 15 kV (for measuring Al, Ba, Ce, La, Mg,Si, and Zr) or 25 kV (for measuring Pd, Rh, and Pt). In EPMA mapping,the element of the substrate component was regarded as Si, and thecatalyst layer components were regarded as Ce and Zr. For SEM fordetermining the width of the division, the observation magnification wasthe same as that for EPMA mapping, and the acceleration voltage was 15kV. PictBear was used as the software for drawing boundary lines. Theselective threshold was set to 30 with reference to the color of a clearvoid section. The results are shown in Table 1. In Examples 1 to 9, theaverage value of the circular void percentages of 10 fields-of-view forthe first catalyst portions was from 12 to 30%, and the average value ofthe circular void percentages of 10 fields-of-view for the secondcatalyst portions was from 10 to 30%.

Mass Ratio CeO₂/ZrO₂, Mass Ratio (CeO₂+ZrO₂)/Al₂O₃, Content of ZrO₂(Mass %), and Mass Ratio ZrO_(2/)Al₂O₃:

From a section different from the section where the sample S was cut outin each of the exhaust gas purification catalysts obtained in Examples 1to 9 and Comparative Examples 1 to 8, a sample portion was cut out thatextended from the downstream-side end face toward the upstream side by alength of 20% based on the catalyst's entire length in the direction X.The composition of the second catalyst portions was determined on thesample according to the aforementioned method. The results are shown inTable 1. Nitric acid and aqua regia were used for dissolving thecatalyst.

Also, in each Example, the pore volume resulting from pores having apore diameter from 5 to 500 nm in the catalyst layers was measuredaccording to the aforementioned method. As a results, the pore volumewas within a range from 0.020 to 0.20 cm³/g in all of Examples.

Further, pressure loss, the peeling rate of the catalyst layer, and thePM collection rate were evaluated according to the following methods.The results are shown in Table 1.

Peeling Rate:

A circular-cylindrical sample cut out from the exhaust gas purificationcatalyst 10 along the exhaust gas flow direction was used as in thedetermination of the circular void percentage. From thecircular-cylindrical sample, the sections extending by a length of 10 mmfrom both end faces, respectively, in the longitudinal direction(matching the exhaust gas flow direction X) were cut off with a hacksaw,to thereby expose the measurement surfaces on the upstream side anddownstream side. Then, a sample having the upstream-side measurementsurface (hereinbelow also called “sample T”), which was the sectionextending by a length of 30 mm from the upstream-side measurementsurface, and a sample having the downstream-side measurement surface(hereinbelow also called “sample B”), which was the section extending bya length of 30 mm from the downstream-side measurement surface, were cutout. A peeling test was performed by placing the sample T with theexhaust gas flow direction oriented horizontal, and blowing air towardthe upstream-side measurement surface of the sample T from the position10 cm away from the upstream-side measurement surface with an air gun at6 atmospheres (gauge pressure) for 10 seconds. The same peeling test wasalso performed on the downstream-side measurement surface of the sampleB. The weight reduction (=(Weight before air blow−Weight after airblow)/Weight before air blow×100 (%)) was calculated for each of thesamples T and B, and then the average was calculated. An average of lessthan 3% was rated A, 3% or greater to less than 6% was rated B, 6% orgreater to less than 10% was rated C, and 10% or greater was rated D.

Pressure Loss:

The exhaust gas purification catalyst 10 was fixed by supporting itsside surfaces with its upstream end face, in the exhaust gas flowdirection, faced upward. Air was sucked downward from below the exhaustgas purification catalyst 10 (i.e., from below the downstream-side endface) at a rate of 50 L/sec. The difference between the air pressure onthe upstream-side measurement surface of the sample T and the airpressure on the downstream-side end face of the sample T 10 secondsafter starting suction was determined, which was taken as the pressureloss. In the evaluation, the acceptance (passing) level was set to apressure loss as strict as less than 15 mmHG.

PM Collection Rate:

A gasoline engine vehicle in which the exhaust gas purification catalyst10 was included was driven in accordance with the driving conditions ofthe Worldwide-harmonized Light vehicles Test Cycle (WLTC). The number ofPM particles in the exhaust gas that passed through the exhaust gaspurification catalyst 10, PN_(cat), was counted in each of the followingdriving phases: low speed phase (from 0 to 589 seconds after the startof driving); medium speed phase (from 589 seconds to 1022 seconds afterthe start of driving); high speed phase (from 1022 seconds to 1477seconds after the start of driving); and extra-high speed phase (from1477 seconds to 1800 seconds after the start of driving). The number ofPM particles directly exhausted from the engine, PN_(all), was alsocounted. The PM collection rate was calculated using the followingequation:

PM collection rate (%)=100-(PN_(cat)/PN_(all))×100.

Conditions for Determining PM Collection Rate:

Vehicle used for evaluation: 1.5-L direct injection turbo engine.

Gasoline used: Fuel for certification test.

Device for counting PM: From Horiba, Ltd.

TABLE 1 Ratio of Circle thickness of (ZrO₂ + equivalent Number ofcatalyst Circular CeO₂/ CeO₂)/ ZrO₂/ diameter of approximately- portionvoid ZrO₂ Al₂O₃ ZrO₂ Al₂O₃ approximately- perfect circular to thicknessPressure PM percentage mass mass content mass perfect circular voids perof partition loss Peeling collection (%) ratio ratio (mass %) ratio (μm)1 mm² wall (%) (mmHg) rate rate (%) Example 1 15.3 0.20 5.24 60.4 4.3517.7 616 10.2 12.1 A 93.1 Example 2 22.6 0.20 5.14 61.1 4.30 18.1 87411.0 10.5 A 91.2 Example 3 28.9 0.20 5.23 61.0 4.36 17.9 1112 11.9 10.1B 90.1 Example 4 11.1 0.19 5.30 60.0 4.46  4.1 3871 10.1 13.1 A 92.1Example 5 12.5 0.19 5.25 60.9 4.42 30.2 256 9.5 12.5 C 91.0 Example 614.4 0.00 5.55 71.4 5.55 18.5 457 15.4 12.1 A 90.1 Example 7 15.1 0.035.80 65.8 5.64 18.3 516 11.5 11.1 A 89.9 Example 8 13.3 0.56 3.12 40.92.13 19.3 427 12.4 9.8 C 89.0 Example 9 14.2 0.00 54.5 81.8 54.5 18.3603 9.8 14.5 A 92.5 Comparative 32.9 0.19 5.19 60.3 4.38 18.0 1456 12.39.8 D 89.2 Example 1 Comparative 0.0 0.18 5.48 61.6 4.65 — 0 6.3 20.8 A90.1 Example 2 Comparative 5.1 0.20 5.11 61.2 4.27 17.2 286 9.3 15.0 A92.9 Example 3 Comparative 11.8 0.82 2.50 33.3 1.37 17.5 465 23.5 10.7 D90.1 Example 4 Comparative 2.4 0.80 2.43 32.9 1.35  3.5 2062 13.7 17.1 A96.4 Example 5 Comparative 0.0 0.81 2.45 33.7 1.31 — 0 18.0 12.7 D 95.9Example 6 Comparative 0.0 0.81 2.42 33.9 1.33 — 0 17.4 11.8 D 92.4Example 7 Comparative 15.2 0.00 10.9 90.4 10.9 18.5 583 9.1 18.2 A 93.2Example 8

As shown in Table 1, small pressure loss, a small peeling rate of thecatalyst layer, and a high PM collection rate were obtained in each ofexhaust gas purification catalysts of Examples, which had a circularvoid percentage of greater than 10% to 30% or less and a ZrO₂ content ofthe catalyst portions of from 35 mass % to 85 mass %.

Although the exhaust gas purification catalysts of Comparative Examples1 to 3 had similar amounts of ZrO₂ as those of Examples, the catalyst ofComparative Example 1, which had a circular void percentage more than30%, had a high peeling rate, and the catalyst of Comparative Example 2etc., which had a circular void percentage of 10% or less, had highpressure loss. Although the exhaust gas purification catalysts ofComparative Examples 4 and 8 had a circular void percentage within therange of greater than 10% to 30% or less, the catalyst of ComparativeExample 4, which had a ZrO₂ content in the catalyst portions of lessthan 35 mass %, had a poor result of peeling rate, and the catalyst ofComparative Example 8, which had a ZrO₂ content in the catalyst portionsmore than 85 mass %, had too much pressure loss. When the circular voidpercentage was reduced in Comparative Example 4, peeling was suppressed,but pressure loss increased (see Comparative Example 5). In ComparativeExamples 6 and 7, in which an acicular void forming reagent was used,the voids were acicular, which resulted in a peeling tendency.

REFERENCE SIGNS LIST

10: Exhaust gas purification catalyst;

11: Substrate;

14: First catalyst layer;

15: Second catalyst layer;

21: Inflow-side cell;

22: Outflow-side cell;

23: Partition wall.

1. An exhaust gas purification catalyst comprising: a substrate, andcatalyst portions provided in the substrate, the catalyst portionsincluding a plurality of voids; the substrate including: inflow-sidecells, each inflow-side cell being a space having an open end on aninflow side thereof and a closed end on an outflow side thereof in anexhaust gas flow direction; outflow-side cells, each outflow-side cellbeing a space having a closed end on an inflow side thereof and an openend on an outflow side thereof in the exhaust gas flow direction; andporous partition walls, each porous partition wall separating theinflow-side cell and the outflow-side cell from each other; the catalystportions being provided on surfaces of the partition walls that eachface the inflow-side cell and/or surfaces of the partition walls thateach face the outflow-side cell; wherein in a cross section vertical tothe exhaust gas flow direction, the percentage of a total area of thevoids, each void satisfying the expression L/{2(πS)^(1/2)}≤1.1, whereinL is a perimeter of the void in the cross section and S is an area ofthe void in the cross section, is greater than 10% to 30% or less basedon an apparent area of the catalyst portion present on the partitionwall; and a content of zirconium element in terms of oxide (amount ofZrO₂) in the catalyst portions is from 35 mass % to 85 mass %.
 2. Theexhaust gas purification catalyst according to claim 1, wherein a massratio of a content of cerium element in terms of oxide (amount of CeO₂)to a content of zirconium element in terms of oxide (amount of ZrO₂) inthe catalyst portions, CeO_(2/)ZrO₂, is 0.8 or less.
 3. The exhaust gaspurification catalyst according to claim 1, wherein a mass ratio of thecontent of zirconium element in terms of oxide (amount of ZrO₂ amount)to a content of aluminum element in terms of oxide (amount of Al₂O₃) inthe catalyst portions , ZrO₂/Al₂O₃, is 1.5 or greater.
 4. The exhaustgas purification catalyst according to claim 1, wherein a mass ratio ofa total of the content of zirconium element in terms of oxide (amount ofZrO₂) and the content of cerium element in terms of oxide (amount ofCeO₂) to the content of aluminum element in terms of oxide (amount ofAl₂O₃) in the catalyst portions, (ZrO₂+CeO₂)/Al₂O₃, is 3.0 or greater.5. The exhaust gas purification catalyst according to claim 1, whereinan average value of a circle equivalent diameter of the voids that arepresent on the partition wall and satisfy said expression is from 1 to60 μm.
 6. The exhaust gas purification catalyst according to claim 1,wherein, in the catalyst portions present on the partition wall, anaverage value of the number of voids satisfying said expression is from250 to 4000 per 1 mm² of apparent area in a cross section of thecatalyst portion.
 7. The exhaust gas purification catalyst according toclaim 1, wherein the catalyst portions include: first catalyst portionseach being provided on the surface of the partition wall that faces theinflow-side cell, each first catalyst portion being formed in at least aportion in the area extending from an upstream end by a length of 20 mmdownstream therefrom in the exhaust gas flow direction; and/or secondcatalyst portions each being provided on the surface of the partitionwall that faces the outflow-side cell, each second catalyst portionbeing formed in at least a portion in the area extending from adownstream end by a length of 20 mm upstream therefrom in the exhaustgas flow direction.
 8. The exhaust gas purification catalyst accordingto claim 7, wherein: the first catalyst portion's length in the exhaustgas flow direction is from 10 to 80% based on an entire length of thecatalyst; and the second catalyst portion's length in the exhaust gasflow direction is from 30 to 90% based on the entire length of thecatalyst.
 9. An exhaust gas purification device comprising two or moreexhaust gas purification catalysts arranged along an exhaust gas flowdirection, wherein: the exhaust gas purification catalyst according toclaim 1 is a second or subsequent catalyst from an upstream side.